This invention relates to vehicle emissions control systems and more particularly to methods and apparatus for determining NOx storage in lean NOx traps (LNTs).
As is known in the art, conventional lean burn engine control systems include an air/fuel controller that delivers fuel to the engine intake proportional to measured air mass to maintain a desired air/fuel, lean of stoichiometric. The typical three-way catalytic converter provided in the engine exhaust passage does not convert the NOx produced while running lean and in order to reduce NOx emission to the atmosphere, it has been proposed to locate a NOx trap (LNT) downstream of the three-way catalyst.
A typical NOx trap utilizes alkali metal or alkaline earth materials in combination with platinum in order to store or occlude NOx under lean operating conditions. The mechanisms for NOx storage involves the oxidation of NO to NO2 over the platinum followed by the subsequent formation of a nitrate complex with the alkaline metal or alkaline earth. Under stoichiometric operation or operation rich of stoichiometric, the nitrate complexes are thermodynamically unstable, and the stored NOx is released and catalytically reduced by the excess of CO, H2, and hydrocarbons (HCs) in the exhaust.
Accordingly, in the prior art, the amount of NOx introduced to the trap since the last purge is estimated and when the trap is estimated to be full the engine is switched to a relatively rich air/fuel to purge the NOx trap. After a predetermined purge time interval, the engine is returned to lean operation. The prior art relies on engine speed to determine NOx accumulation, see for example U.S. Pat. No. 5,473,887. However, engine speed alone does not provide an accurate estimation of NOx accumulation since several other variables that affect NOx accumulation are different at the same engine speed depending on other engine operating condition, thereby causing wide swings in the rate of NOx accumulation. It is important to obtain as accurate an estimate of NOx accumulation as possible since underestimation will permit lean operation to continue after the trap is full and tailpipe NOx emission will result. On the other hand overestimation of the accumulated NOx will cause purging at a rich A/F at a higher frequency than required, reducing fuel economy.
Other methods have been described for maintaining catalyst efficiency of NOx traps and for monitoring the performance of NOx traps. See for example, U.S. Pat. No. 5,894,725 xe2x80x9cMethod and Apparatus for Maintaining Catalyst Efficiency of a NOx Trapxe2x80x9d inventors Cullen et al. Issued Apr. 20, 1999 and U.S. Pat. No. 5,771,685. xe2x80x9cMethod for Monitoring the Performance of a NOx Trapxe2x80x9d, inventor Hepburn, issued Jun. 30, 1998, both assigned to the same assignee as the present invention, the entire subject matter of both such U.S. Patents being incorporated herein by reference.
More particularly, during lean operation, the NOx is absorbed in a Lean NOx Trap by the NOx storing element, for examplexe2x80x94Barium (Ba) by the following chemical reaction:
BaO+NO+O2xe2x86x92Ba(NO3)2
As can be seen from the equation above, the NOx is absorbed using oxygen in the exhaust as part of the chemical reaction. Since oxygen is getting used up for absorption, the change in the oxygen concentration between the exhaust air/fuel entering the LNT and exiting the LNT can be used to determine the amount of NOx absorbed in the LNT. The inventors have recognized, however, that there is a water gas shift reaction that occurs in the exhaust which converts CO to H2 in the vehicle""s exhaust system given by: CO+H2Oxe2x86x92H2+CO2. Thus, the oxygen is used in both the NO reaction and in the hydrogen reaction. Therefore, in order to obtain an accurate determination of the amount of NOx in the LNT, the measurement of the change in the oxygen concentration between the exhaust air/fuel entering the LNT and exiting the LNT must be corrected for the water gas shift reaction effects. Absent such correction, the water gas shift reaction causes the tailpipe air/fuel ratio to be richer (less leaner) than the true value, thereby over estimating the amount of NOx absorbed in the LNT.
Further, the inventors have recognized that the amount of hydrogen used in the water gas reaction is related to the mathematical difference between an air/fuel ratio upstream of the LNT and an air/fuel ratio measured downstream of the LNT.
In accordance with the present invention, a method is provided for estimating the amount of NOx stored in a NOx trap. The method comprises determining a change in the oxygen concentration between an exhaust air/fuel ratio entering the NOx trap and exiting the NOx trap to determine the amount of NOx absorbed in the trap.
In one embodiment, the method includes correcting such determination for water gas shift reaction effects in such change in oxygen concentration determination.
With such method, the estimation of the amount of NOx stored in the trap is corrected by correcting for the effect of hydrogen and water gas shift reaction in the trap. During a NOx purge cycle, the engine is first run at a rich air/fuel ratio to purge NOx from the trap. Then the engine is subsequent placed in a lean air/fuel ratio mode; i.e., the engine is commanded to run at a predetermined lean air/fuel ratio, A/Fc typically 1.4 times nominal stoichiometry. Over an initial transient period of time, T, after initiating of this lean air/fuel ratio mode, the tail pipe, or downstream, air/fuel ratio reaches a steady state condition A/Fss. The integration of the measured downstream air/fuel ration over the period of time T represents the integrated amount of oxygen concentration difference between oxygen entering and exiting the trap. This includes the amount of oxygen used in reacting with the NOx and the amount of oxygen reacting with the hydrogen in the water gas shift reaction. However, once the trap filled with NOx, the tail pipe, downstream air/fuel ratio reaches a steady state difference of xcex94. This is due to water gas shift reaction caused by hydrogen in the trap. The error, i.e., xcex94=A/Fcxe2x88x92A/Fss times the period of time T is A2=xcex94*T. Thus, the correct amount of oxygen used to react with the NOx and thus representing the amount of NOx absorbed by the trap is represented A=A1xe2x88x92A2. Thus, this invention improves the methodology to give an accurate estimation of the NOx absorbed in the trap.
In accordance with a feature of the invention, a method is provided determining the amount of hydrogen used in a water gas reaction in a device, such as a NOx trap or a catalytic converter, disposed in the exhaust of an engine. The method includes determining a mathematical difference between an air/fuel ratio upstream of the device and an air/fuel ratio measured downstream of the device.
In accordance with the invention, a method is provided for determining the amount of hydrogen used in a water gas reaction in a reaction device disposed in an exhaust of an engine comprising determining a mathematical difference between an air/fuel ratio upstream of the device and an air/fuel ratio measured downstream of the device.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.